FR2475803A1 - HETERO-JUNCTION LIGHT EMITTING DIODE - Google Patents

Abstract

A. HETERO-JUNCTION LIGHT EMITTING DIODE. B. LIGHT EMITTING DIODE CONTAINING ON A SUBSTRATE 1 A REGION 2 A SEMICONDUCTOR OF TYPE P HAVING A CONCENTRATION OF CARRIERS P SUCH AS: 4.510CMP2.510CMET A REGION 3 A SEMICONDUCTOR OF TYPE N HAVING A BAND INTERVAL PLUS LARGE THAN THAT OF THE P-TYPE REGION AND A CONCENTRATION N IN CARRIERS SUCH AS: 210CMN110CMET WHICH CONSTITUTE THE PN 6 HETERO-JUNCTION WITH THE TYPE PC REGION 2 THE INVENTION APPLIES IN PARTICULAR TO LIGHT-EMITTING DIODES.

Description

The present invention relates to a semi-radiating device

  conductor, and more particularly to a light-emitting diode having a hetero-junction and

  emitting incoherent light.

  The devices-. Solid state radiators typically consist of compound semiconductors or compound crystal semiconductors. Manufacturing processes impose different limitations on the structure of these semiconductor devices. Formation of a doped region is usually done by diffusion

  or by epitaxial growth in the liquid phase.

  We will now describe a certain

  number of examples of conventional light emitting diodes.

  A GaAsP light emitting diode consists of an n-type GaAs substrate, an n-type GaAsP epitaxial layer formed on the n + type GaAs substrate, and a p-type region that is selectively diffused in the n-type GaAsP layer.

  GaAsP of type n. The p-type diffusion region has a concentration

  typical of impurity on the order of 1 x 10 18 cm 3 and is thin so as to be below several micrometers to prevent excessive light being absorbed in this region of type p. The outgoing light is extracted from the face of this region of

  p type located opposite the substrate.

  A GaP light emitting diode is

  consisting of an n + type GaP substrate, an epitaxial

  xiale n-type GaP, and a p-type epitaxial GaP layer, these two last epitaxial layers being successively grown on the n-type GaP substrate. The p-type layer has a typical impurity concentration of about 1 × 1018cm * The outgoing light is extracted on the side of this p-type layer. A GaAs light emitting diode has a structure similar to that of the previously mentioned GaP light emitting diode. P-type surface layer at a typical impurity concentration of about 2 x 10 8 cm. The outgoing light is extracted from both sides of the p-type surface layer or on the side of the p-type surface layer.

  n + type substrate (in the latter case the substrate is partially

  pickled where the outgoing light is to be extracted). A GaAlAs light emitting diode consists of a p + type GaAs substrate, a p-type Ga1 AlxAs epitaxial layer formed on the GaAs substrate, and an n-type Ga 1 AllyAs epitaxial layer formed on the epitaxial layer.

  type p. The mixture ratios x and y are chosen for

  satisfy the condition x <y so as to effectively extract the outgoing light on the side of the n-type epitaxial layer. The n-type epitaxial layer has a typical concentration of

  impurity of the order of 1 x 10 17 cm 3.

  It should be understood that there are many types of light emitting diodes and different considerations need to be taken into account.

for these different structures.

  Usually in compound III-V compound semiconductor light emitting diodes it is easier to obtain better emission efficiency from

  light in p-type regions only in n-type regions.

  Light-emitting diodes use spontaneous radiation emission in contrast to stimulated radiation emission in semiconductor lasers. As a result, the lifetime of the minority carriers in a light-emitting diode is relatively long. Minority carriers injected into a radiative region through a pn junction have a large possibility of capture by

  non-radiative recombination centers and to be recom-

  bined with majority carriers without emitting light rays. It is therefore very important to reduce the non-radiative recombination centers in the radiative region of a diode

light emitting.

  Network faults are largely responsible for

  sands of the production of non-radiative recombination centers.

  Usually one of the p-type regions and n-type regions constitutes a pn junction at a lower density of non-radiative recombination centers and constitutes a predominantly radiative region. In the GaAs and Ga Al xAs case, defects are more likely to form in an n-type region than in a p-type region. In an n-type region doped with a donor impurity such as Te, Se or S, gaps may be formed which constitute a compensation.

3 2475803

  for the donor atoms. The density of such

  gaps is considered to be proportional to the concentration

  doping. Such gaps and / or combinations of gaps and donor lmp-residues are considered to be effectively n-on radiative centers. In a light emitting diode having a diffused homo junction, zinc (Zn) is usually diffused as an acceptor impurity in an n-type crystal. It is very difficult to diffuse a donor impurity in a p-type crystal, since there is no donor impurity with a large diffusion constant like those

zinc.

  The diffusion of zinc naturally makes the concentration of carriers in the p-type region greater than that of the n-type region. The concentration of the diffused zinc should not be so high as to maintain good crystal properties of the p-type scattered region. The n-type region within which the zinc is diffused must have a lower impurity concentration than that in the p-type diffusion. As a result, it is difficult to increase the injection of minority carriers into the radiative region.

through the pn junction.

  Light emitting diodes manufactured

  by liquid phase epitaxy are generally effective.

  higher light emission than the diodes emitting

  scattered light. Epitaxial layers have

  Fault densities lower than those of the scattered regions.

  In addition any combination of impurity concentration can

  be selected for a pn junction grown epitaxially.

  Accordingly, in a pn-homo epitaxial diode, the carrier concentrations of the respective regions are chosen to maximize the efficiency of the minority carrier injection into the radiative region. For example, in a light-emitting diode having a p-type radiative region, the carrier concentration of the p-type region is chosen to be sufficiently low compared to that of the n-type region to increase electron injection into the region. of type p with respect to the injection of trcus into the n-type region. In all cases the radiative region is expected to have a low impurity concentration in order to increase the efficiency for

4 2475803

  injection and efficiency for light emission.

  The light emitting diode

  The junction has the advantage of producing a window effect as described above and may have a higher external efficiency than the homo-junction light emitting diode. The hetero-junction diode may have other advantages over the homo-junction light-emitting diode, but these superiorities have not been fully exploited. An object of the present invention is to create a hetero-junction light emitting diode

  exhibiting excellent external quantum efficiency.

  Another object of the present invention is to provide a hetero-junction light emitting diode of the type described above, which has optimal carrier concentrations in the respective p-type and n-type regions.

which constitute the junction p n.

  Another object of the present invention is to create a hetero-junction light emitting diode of the type described above which has a radiative region of a

optimal thickness.

  In a hetero-junction light emitting diode, the p-type region and the n-type region may have different slope intervals. In such a case, the efficiency of the injection into the respective p-type and n-type regions is mainly determined by the difference in the band intervals, and not by the carrier concentrations in the respective p-type regions. and of type n. The author of the present invention has considered that the radiative region must have a high concentration of impurity to increase the concentration of the radiating recombination centers. An excessively high concentration of impurity however would increase the various defects of the crystal such as the gaps and would lower the efficiency of light emission. Consequently, there must be limits

  optimal carrier concentration for the

  nante. The other region, to inject minority carriers

  in the radiating region preferably has a concentration

  carriers to lower the resistivity and to inject a sufficient quantity of minority carriers

2475803

  the radiative region. An excessively high concentration of impurity, however, would increase the various defects of the

  crystal and would lower the efficiency of light emission.

  Accordingly, there must also be an optimum range of carrier concentration for the injector region. In the GaAs and GaAlAs light-emitting diodes the predominantly radiative region is preferably a p-type region since the defects of the grating are more likely to form in an n-type region than in a p-type region. The impurity of type p can be zinc (Zn), germanium (Ge), etc. Among the various impurities of type p, zinc gives the best emission efficiency. Zinc, however, has a large diffusion constant and can therefore diffuse into the n-type region during cultivation. If the zinc diffusion in the n-type region is intense, the part of this n-type region adjacent to the p-type region can be inverted in the p-type and the location of the p-and n-junction can be modified. Such a displacement of the junction p n would change the wavelength of the emission into a shorter length and transform the hetero-junction p n into a homo-junction p n. This factor also imposes an upper limit on carrier concentration in p-type regions.

  In agreement with the results of the experi-

  mentation conducted by the author of the present invention, he

  external emission efficiencies have been found to be the best

  are obtained when the p-type region has a concentration

  tration p of carriers within the limits of 4.5 x 107 <p <2.5 x 10 18 (cm-3) and when the n-type region has a n-type concentration of carriers within the range of

  limits of 2 x 1017, n <1 x 10 8 (cm3).

  In a light-emitting diode the diffusion length of the minority carriers can become large. A thin radiative region is insufficient for most minority carriers to recombinate. From this point of view, the thickest is the radiative region. the highest is the efficiency of the show. However, there is also a loss of internal absorption and optimal conditions for reducing network faults, etc., which may impose an upper limit for the thickness of the radiative region. In

6 2475803

  consequently the optimal limits for the thickness of the radiative region have been studied experimentally. In agreement with the results of these experiments, the efficiency of the emission increases as the thickness of the radiative region increases to at least 40 μm. The thickness of the radiative region is preferably equal to or greater than 10 μm to increase the efficiency of the emission and is preferably less than 40 μm.

  considerations of manufacturing processes and savings.

  Other objects of other features and advantages of the present invention will result from the

  following description of preferred embodiments

  shown in the accompanying drawings in which FIG. 1A is a diagrammatic view in FIG.

  partial section of a hetero-junction light emitting diode

  FIG. 1B is an energy deviation diagram of the light emitting diode according to FIG. 1A; FIGS. 2A and 2B are respectively diagrams of the carrier concentration and the energy deviation in a part of the FIG. light emitting diode of FIG. 1A; FIGS. 3 and 4 are graphs showing the relationship between gloss and carrier concentration; FIG. 5 is a graph showing the relationship between brightness and thickness of the layer; FIG. 6A is a schematic partial sectional view of a light emitting diode modified with respect to FIG. 1A; FIG. 6B is a diagram of energy gaps corresponding to the light emitting diode of FIG.

Figure 6A.

  Figure 1A shows a heterojunction GaAlAs light emitting diode. On a p-type GaAs substrate, epitaxially a Ga xAlxAs p-type layer 2 is developed epitaxially. An n-type Ga1 _Alias layer is developed epitaxially on this p-type layer 2 to form a pn hetero-junction. , 6. x and y are chosen to satisfy condition x. in order to achieve a band gap profile such as that shown in FIG. 1B so that the n-type layer 3 is transparent to the light emitted in the p-type gate 2. Indeed, the n-type layer 3 has a wider band interval than the p-type layer 2 and

  the light emitted in the p-type layer 2 can be effectively

  The layer 2 of type p has a certain absorption coefficient for the light emitted therefrom. Therefore, the more the location of the light emission will be close to the pn junction, the lower will be the absorption undergone by the light thus emitted. In the structure shown in FIG. 1A, the concentrations of carriers of the layer 2 of p-type and n-type layer 3 were varied in a variety of ways to find an optimal combination of carrier concentrations in these respective layers 2 and 3. Uin epitaxial growth from a solution was used to form a

hetero-junction p n.

  8pitaxial development is advantageous

  to form respective layers of electronic properties

  uniforms and good crystal properties. Around the junction, however, there is a mutual atomic diffusion which to a certain extent

  gradient of impurity concentration.

  Figures 2A and 2B show diagrammatically

  the distribution of the impurity concentration (FIG. 2A) and the distribution of the band interval around the pn junction 6 of FIG. 1A. The abscissae represent the distance in the direction normal to the pn junction and are recorded on the FIG. 2A and FIG. 2Bo. The ordinates of FIG. 2A represent the difference ND-NA between the concentration ND of the donor and the concentration NA of the acceptor. The concentration of the donor doped initially in layer 3 of type n is

  MD0 while the concentration of the acceptor initially doped-

  In p-type region 2 is NA0O During epitaxial merging, p-type and n-type impurities are diffused. The resulting donor distribution is represented by a dashed line and the distribution of the acceptor which is result is represented by a curve in

8 2475803

  Dashes 12. As a result, the total resulting net distribution of impurities is that represented by curve 10. The atoms constituting the semiconductor are subjected to a lower diffusion rate. As a result the resulting distribution of the band interval is steeper than the distribution

  impurity as shown in curve 14 of FIG. 2B.

  When the acceptor (eg zinc) diffuses into the n-type region 3 through the pn junction 6, the concentration of the acceptor in the p-type region 2 portion adjacent to the pn junction decreases. In addition, the donors diffused in the p-type region 2 constitute a

  compensation for acceptors in this region. As a result

  the magnitude of the net impurity concentrations around the pn junction decreases but the displacement of the

  pn junction occurs only in a relatively difficult way.

  However, if the concentration of zinc in the p-type region 2 chosen too high compared to the concentration of the donor in the n-type region 3, the pn junction can be displaced within the initially n-type region 3 which has a wider band interval. This leads to the emission of a shorter light and also to a decrease in the efficiency of the emission due to an increase in the

  electron density at the edge of the indirect conduction band.

  The experiments are conducted in such limits as the displacement mentioned above of the

  pn junction does not occur or is quite negligible.

  Epitaxial growth is effected by the method of

  The temperature range proposed in Japanese Patent No. 857545 which corresponds to US Patent Application No. 871113. The epitaxial growth by gradual cooling of the solution is accompanied by variations in composition,

  impurity concentration and in the rate of non-stoichiometry.

  As a result, the impurity and band gap distributions can not be so flat as shown in FIGS. 2A and 2B and also the defects in the areas can not be reduced particularly well. Therefore, the relationship between brightness and impurity concentrations in the respective regions can not be clearly observed. Moreover, in accordance with the temperature difference method, a high temperature feed portion and a portion of

9 2475803

  low temperature growth are maintained at temperatures below

  respective constant rates. The raw material dissolves in the feed part, diffuses through the solution and grows in the growth part. As a result, the epitaxial growth is carried out at a constant growth temperature from a solution of constant composition. In this way an epitaxial layer of uniform properties can be obtained. By using a plurality of solution crucibles and a slider-carrying substrate, successive multiple epitaxial growths can be made. With the exception of the limit pn0, the atomic diffusion of the solid phase modifies the impurity concentration and the composition as shown in FIG. 2A and FIG. 2B, the variations in the respective composition of Ga1x layer Ai As and of the 3 Gal AlyAs layer, i.e. x and Ay, can be easily reduced so as not to be greater than 0.01 and can be reduced below 0.002 even if the epitaxial layer has a non-inferior thickness at 10) m. By way of comparison, the composition x of A1 in the GaI Al Al layer developed by the gradual cooling process tends to decrease when the growth time increases, and this tendency is usually accompanied by a variation.

  x of the order of b x - 0.02 for 10 m of thickness.

  The experimental results are shown in FIGS. 3 and 4 in which the abscissas represent the concentrations p and n in carriers in the p-type layer (FIG. 3) and in the n-type layer (FIG.

  the ordinates represent the brightness in millicandelas.

  It can be seen that the gloss is high, ie the emission efficiency is high within the limits of 4.5 x 1017cm-3 <p <2.5 x 10 8cm3 2.0 x 1017cm 3 4 nr 1.0 x 1018 cm Most preferably, the concentrations p and n of carriers can be chosen within the limits of: 8 × 10 -7 cm-3 × 2.0 × 10 -8 cm 3 2.0 × 10 17 cm 3 <δ 890 × 10 17 cm -3

2475803

  The following carrier concentration limits 8.0 x 10lcm-3p 2.0 x 1018cm3 4.0 x 1017cm3 A n -: 8.0 x 1017cm 3

are also preferable.

  It is also preferable that the acceptor concentration p be chosen greater than the

  concentration of donors n, p> n, to decrease the den-

  of defects in the n-type region. It must be emphasized

  here that the concentrations p and n of carriers mentioned below

  above correspond to the concentrations of acceptors and

  NA0 and NDO donors explained in connection with Figure 2A.

  The composition x of the p-type layer Ga xAlxAs 2 was chosen within the limits of 0.30 × z_0.37 1.5 since the composition y of the n-type Ga1lyAlyAs layer 3 was chosen within the limits of 0.40 c 0.70 y. When the aluminum composition exceeds the level of about 0.40, the indirect transition is dominant and lowers the efficiency of the emission. However, when the aluminum composition is below 0.30, the emission wavelength shifts to a region of low visual sensitivity. The composition can be determined by X-ray microanalysis, but the value obtained can vary depending on the calibration method used. The composition can be determined more precisely by measuring the peak wavelength of the emission. The relation between the photon energy hd and the composition x in the Ga1 xAlxAs direct transition region can be represented by: hi (eV) = 1.371 + 1.429x The photon energy h 'and the emission wavelength 7 (A) are related by the equation

1 = 12400 / h-) (A).

  Between the narrow limits 6550A; e) k, 6900A

  the relationship between the emission wavelength and

  can be represented approximately by:

7 (A) = 8468-5260x.

  The dispersion of the peak wavelength observed in the emission spectrum was less than + 5A (+ 0.0015eV) around 6650Ao. This value corresponds to a varation of less than 0.002. The carrier concentration can be measured for example by forming contacts

  of Schottky on a folded surface at an angle.

  We will now describe the effect of thickening

  of the xregion radia.tx-e. If the radiant region has no absorption for the light emitted, the thickness of this radiative region is preferably much greater than the diffusion length L = D. an epitaxial layer developed by the gradual cooling method a a lifetime of the minority carriers of the order of 3 ns zu minus and a constant -9 diffusion D of the order of 100 cs / s oU m0oDS thanks to what

  the diffusion length was estimated to be around 5 m.

  To induce the recombination of the majority of the minor carriers

  rites injected into the regio! radiating this radiant region

  It should be much thicker than the diffusion length. However, the light that is emitted inside the radiative region undergoes internal absorption and the growth of a very thick layer is difficult by the gradual refrolding process and makes the transmitting diode

  expensive light. Usually the ipai3seur of the region ray-

  nantea was chosen in the order of 1 to 2 yvm.

  according to the process of difference of temperature

  epitaxial layers with good crystal properties can be obtained. For example, the lifetime of the minority carriers may be of the order of t0 ns and the diffusion constant may be of the order of 300 cm 2 / s. The author of the present invention studied experimentally the dependence

  between the efficiency of emission and the thickness of the

  -0 p-type nant developed by the temperature difference method

  The experimental results on light-emitting diodes having a peak emission wavelength of 66C50A are shown in Figure 3. The diodes

  emitters of a range of transmitters of

  viron 6550 to 6900 A shows also a similar trend

as shown in Figure 5.

  Fig. Shows that at least up to about 40, a higher emission efficiency is obtained from: The radiant region is much larger. he

12 2475803

  It should be emphasized that the region ayonnan- is developed by the process of temperature difference and presents

  very uniform properties throughout its thickness.

  When the thickness is less than m the emission efficiency decreases rapidly. Between about 5 Am

  and about 17 m the efficiency of emission increases with the

  but shows a tendency to saturation. There is a kind of knot around 17lm. Above 17 m the emission efficiency increases again. Above about 30 m again some degree of saturation tendency is observed. Therefore it is desirable to choose for the thickness of the radiative region at least about 5m, preferably more than 10μm, and more preferably more than 17km. Thicknesses greater than 40>,% m, at least in the present state of the manufacturing technique, are not easy to

  realize and are not economical.

  At the optimum thickness, impurity concentration and composition conditions, the GaAlAs light emitting diodes molded with epoxy resin were noted to have an external emission efficiency of about 4% (internal emission efficiency). about

27%).

  The rapid decrease in emission efficiency below a thickness of about 5 μm may be

considered as follows.

  Although the Gay xAlxAs network is adapting

  well with that of GaAs compared to other heterogeneous

  such as GaAs-GaAs lxP x, the GaAs substrate usually

  It has a lower purity and a higher defect density than the GaAl xAs epitaxial layer. Thus such lattice defects and impurity inputs can be transferred and produce effects in the epitaxial layer beyond the heterojunction to some extent. Excessively thin epitaxial layers may be further influenced by defects and / or impurities in the substrate resulting in poorer emission efficiency. The upper limit of the practical thickness of the étaxial layer

  is determined by manufacturing techniques and cost. A modification of the structure of the light emitting diode and which

Achieves efficiency

13 2475803

  high emission by a simple method will now be described. When a p-type layer 2 Ga xAl xAs is oppEe on a GaAs substrate, 1, of p + type which has a much higher acceptor concentration than that of the epitaxial layer 2, the atom atoms in the substrate 1 diffuse in the p + type Ga1_x AlxAs epitaxial layer to form a thin p-type GaAl Al 8 Ga layer as 1-xx shows Figure 6A. The p + type GaAs Al As 8 layer has a higher potential than the p-type Ga 1AlAlAs layer for electrons as shown in FIG. 6B. The potential difference represented by q in FIG. 6B serves to reflect electrons injected from the n-type Ga _yAlyAs layer, 3, through the pn junction 6. The reflected electrons may have the possibility of radiative recombination in the layer Galx Ai As, 2, type p. The p + type Ga x Al x As 8 layer can be formed, of course, by epitaxial growth, but it is much better that it be formed using diffusion from the substrate without increasing the number of manufacturing operations as described. above. To cause such diffusion, the substrate must be doped with an acceptor having a large diffusion constant. So it is better to use a

substrate type p + dope zinc.

  The potential barrier formed by the difference in carrier concentration is not very high. So it is desirable that the thickness of the layer Ga Ai As, 8, of type p + is not much smaller 1-x x

  as the diffusion length inside this layer.

  The height of the potential barrier is determined by the concentration of acceptors p of the highly doped layer Ga xAl xAs, 8, and the concentration

  of acceptors p of the Ga xAlxAs layer, 2, developed epitaxially

according to the relationship:

3 = KT ln (p + / p).

  About half of the electrons avoid diffusion when

p + 2.7p ......... (1).

  and most electrons avoid diffusion when p 7.5p ......... (2)

1 4 2475803

  in agreement with the Maxwell'Boltzmano distribution If the impurity of the GaAs substrate, 1, of the p type is zinc, the impurity concentration in the part of the Ga1AlAs layer, 2, adjacent to the substrate 1 is very close that of the substrate 1. For example when the substrate

  GaAs, 1, of type p has a lower zinc impurity concentration

  than 1 x 1019cm -3, and when a Ga1 Al As layer, 2, of

1-X X 8 3

  p-type having an acceptor concentration of about 1 x 1018cm is developed on this substrate, a layer Ga Al As, 8, of -X p + type satisfying condition (2) can be produced. It should be noted that this effect only occurs when the epitaxial layer has a uniform composition since a potential barrier of about 46 meV corresponds to a difference in

x composition of about 0.03.

  Although the description

  limited number of exemplary embodiments, the scope of the present invention is not limited to these embodiments. For example a small amount of phosphorus (P) can be added to the

  type p and / or the n-type region to further improve the adaptation

  networks without substantial changes in other physical properties. Better adaptation of

  networks will reduce network faults such as dislocations

  unsuitable for hetero-junction and improves the efficiency of emission. Other modifications and combinations of the described embodiments are obvious to those skilled in the art.

1 5 2475803

Claims (9)

  1.- Diode emitting light to hetero   jonctiorn, a light-emitting diode, characterized in that it comprises a compound semiconductor semiconductor hetero-junction of the group 1-11-V mainly composed of Ga or Ga and Al as elements of group IIi and As as element of group V, this light-emitting diode comprising:   = a p-type semiconductor region having a concentration   of carriers p within the following limits: 4.times.10.sup.-3 cm.sup.-2.5 x 1018 cm.sup.3 - an n-type semi-conductor region having a wider band gap than that of the p-type region and a concentration of carriers n within the following limits g 18 3   2 x 101 cm3 n 1 x 10 cm and constituting the pn heterojunction with the p-type region.   2.- LED emitting light to hetero   junction according to claim 1, characterized in that the   p concentration in carriers of the p-type region is mainly   determined by impurity doping, constituted by   zinc, made in this region.   3.- LED emitting light to hetero   joining according to one of the claims 1 and 2,   characterized in that the carrier concentrations satisfy with the condition p> n.   4.- LED emitting light to hetero   junction according to claim 1, characterized in that it comprises a p-type GaAs substrate having a higher carrier concentration than that of the p-type region, the region   p-type being disposed on this substrate and having a composition   This expression is essentially expressed by a a1 Alx As, the n-type region being disposed on this p-type region having a composition essentially expressing Ga yAlyAs, wherein the heterojunction light emitting diode is claim 4, characterized in that x   satisfies the condition g 03 <'_ x J 0.37. 2475803   6.- LED emitting light to hetero   junction according to claim 1, characterized in that the   region of p has a thickness of at least about 5.   7.- LED emitting light to hetero   junction according to claim 19 characterized in that the   p-type region has a thickness of at least about 10 "m.   8.- Diode emitting light to hetero   junction according to any one of claims 1, 4 and 5,   characterized in that the p-type region has a thickness at less than 17 m.   9.- Light emitting diode has hetero   junction according to claim 4, characterized in that it comprises another region of type p essentially having the   same composition as the first p-type region and a concentration   in p + carriers which is disposed between the substrate and the first p-type region, the carrier concentration + p satisfying the condition s + p> 2.7p.   10.- LED emitting light to hetero   junction according to any one of claims 1 and 9,   characterized in that the variation A x of x across the   first region of p satisfies the condition g 4 x 4 0.01.